It is known in the prior art to propagate an optical signal through an electro-optic crystal where a radio frequency, RF field is imposed on the crystal. The RF phase is adjusted to modulate the refractive index of the electrooptic medium to adjust the phase approximately linearly with time for the propagating optical pulse. The decreasing optical path produces an effect similar to that of uniform motion of the optical source toward the observer, e.g., a doppler upshift. A 180.degree. shift in the RF phase creates the effect of uniform motion away from the observer (doppler downshift). See Duguay and Hansen, IEEE Journal of Quantum Electronics, QE-4, p. 477, (1968). The effect is reviewed by Kaminow and Turner, Proc. IEEE, vol. 54, pp. 1122-1124 (1966).
In the present work, we describe a different structure wherein an RF traveling wave in an RF guide structure co-propagates with an optical pulse in an optical guide incorporating an electro-optic medium. The RF phase is adjusted in relation to the co-propagating pulse to produce a desired refractive gradient in the electro-optic medium whereby the optical pulse experiences a uni-directional wavelength shift of desired magnitude. In contrast to the lumped element arrangement of prior art (for which the refractive index is constant in space) the present invention yields a refractive index which is a function of space and time.
A very useful arrangement integrates, on a single substrate, the electro-optical modulator with the RF transmission line for use as a component in particular applications.
Consider a light pulse propagating through a nonlinear medium which exhibits a linear electro-optic coupling. A microwave signal E.sub.m propagates coincidentally with the optical pulse. The microwave field EQU E.sub.m =E.sub.0 sin (kz-.omega.t) Equ. 1
produces a dynamic effect upon the refractive index of the nonlinear medium given by EQU n=n.sub.o +.delta.n EQU .delta.n=-(n.sub.0.sup.3 r.sub.c E.sub.m)/2 Equ. 2
where n.sub.0 is the unshifted refractive index of the medium and r.sub.c is the effective first order electro-optic coefficient. Assume that the light pulse is spatially coincident with the microwave phase as illustrated in FIG. 1. As shown, the pulse is so located in time and space that it coincides with an increasing slope of the field E.sub.m and thus a reduced index of refraction at its trailing edge and an increased index of refraction at its leading edge, both instantaneously and locally. This results in the leading edge of the pulse traveling at a reduced velocity with respect to the trailing edge. As a result, the pulse is spatially squeezed. Although the pulse is spatially squeezed, the total number of wave periods or cycles remains the same, yielding a frequency upshift. A frequency downshift may be obtained by synchronizing the optical pulse with the opposite RF phase, e.g., by locating the optical pulse in a region of decreasing electric field E.sub.m. The leading edge of the pulse then propagates at a relatively greater velocity than the trailing edge because the leading edge experiences a lesser index of refraction. Thus the pulse is spatially stretched, the same number of optical cycles occupying a greater spatial length.
In this discussion, it is sufficient to assume a positive electro-optical coefficient although that nothing herein is intended to limit the nature of the medium or the operating range wherein the coefficient has a specific sense.
In general, the refractive index is a function of the wavelength, especially when comparing optical and microwave radiation. As a consequence the optical propagation velocity (more specifically, the group velocity) is usually different from the phase velocity of the RF wavefront. In one embodiment of the present invention, this difference may be ameliorated by introducing period polarity reversals in the transmission line.
The present work embraces both a uni-directional incremental wavelength shift and bidirectional shifting similar to symmetrical sideband modulation. The uni-directional embodiment of the wavelength shifter of the present invention is useful in a wide range of measurement applications. The symmetrical bidirectional embodiment is also well suited for instrumentation as described herein.
In the prior art there has been an application for controlled wavelength shifting in the optical region in the area of atomic absorption spectrometry. It is known in that work to employ the Zeeman (or Stark) effect to shift the emission wavelength of a probe beam obtained from a hollow cathode lamp or the absorption wavelength of a test sample under the influence of an external field as described by U.S. Pat. No. 4,341,470. An electro-optic wavelength shifter has been described for a similar application by Cammann (U.S. Pat. No. 4,834,535). In the latter work an optical modulator is subject to an applied RF field and the optical beam is caused to repeatedly traverse the modulator. The RF field of the reference necessarily has a wavelength which is large compared to the dimensions of the modulator in contrast to the requirements of the present invention wherein an RF traveling wave locally modifies the optical properties of the modulator to provide a spatial and temporal modulation of the optical properties of the electro-optical medium.
It is known to employ the propogation of optical solitons on an optic fiber to support a communication system exhibiting unusually high bit rates. Such a system was studied and described by Mollenauer, Gordan and Islam, IEEE J. of Quant. Elect., vol. QE-22, pp. 157-173 (1986). A practical system of this type requires multiplexing of different channels on the same physical fiber. A frequency multiplexing and demultiplexing apparatus utilizing the present invention is described below.